電感耦合式氬氣體電漿源不穩定現象之動態特性量測分析研究

Investigation of instabilities in inductively coupled Ar plasma

張家豪(Chia-Hao Chang)

指導教授 ： 柳克強

國立清華大學/原子科學院/工程與系統科學系/博士(2007年)

https://doi.org/10.6843/NTHU.2007.00493

摘要

電漿製程於先進半導體製造工業中扮演著極為重要的角色，而製程的結果往往是由腔體中電漿特性所決定。就商業製造成本的考量，一個穩定的電漿源除了增加製程良率之外，並能降低系統良率損失以提高生產力來降低製造成本。基於應用的廣泛，透過實驗量測以及理論計算的分析，電感耦合式電漿源其產生機制及相關物理特性、化學反應等等電漿參數均有大量且測底的研究。然而，在電漿源的穩定性方面卻鮮少被留意。近年來，諸多研究顯示使用負電性氣體，在低功率及低氣體壓力的條件下，電漿會產生自發性的擾動現象。這類型基於電子捕獲反應(electron attachment reaction)所生成負離子而衍生出的電漿不穩定現象之機制，總歸為attachment-induced instability。然而於此研究中，發現在使用Ar惰性氣體時，電漿也會產生規則的振盪情形，此不穩定現象並無法利用前述之負電性氣體不穩定理論來加以解釋。由於射頻電感式電漿系統中，電漿的產生係受到吸收功率的影響，而腔體中電子密度的多寡及其溫度即決定電漿的阻抗大小。然而，射頻功率的傳輸又受到不同的阻抗(或電子密度)的影響。這之間的交互作用進而影響到電漿系統穩定與否。
因此，本研究研製射頻電漿阻抗計以及傳輸線式微波干涉儀，期針對氬氣體電漿源不穩定現象中，射頻傳輸功率與電漿密度之間的關係進行研究的。首先利用脈衝調變式（pulsed mode）電漿源，其週期性射頻功率以及阻抗的變化特性，來驗證所研製的射頻電漿阻抗計的效能。然而，由於不穩定現象發生在低功率條件下，此時傳輸線式微波干涉儀的訊雜比過高，造成電子密度變化量測結果不甚明顯。因此，改使用具射頻補償功能之蘭牟爾探針來量測電漿中飽和離子電流的變化。
在非穩態電漿的研究中，發現當使用Ar做為工作氣體時，在特定的匹配網路位置範圍內，電漿會發生規則性的振盪情形。而不同的匹配網路設定會分別產生高頻率振盪(10~20 kHz)以及低頻率振盪(150 Hz)兩種情況。前者振盪頻率會隨著匹配電路、射頻功率的設定以及腔體中氣體壓力而改變，而低頻率振盪的頻率則不受上述操作條件的影響。在研究中，利用等校射頻匹配電路以模擬不穩定發生時，電漿密度(系統輸入阻抗)與輸入功率之間的變化關係。結果顯示當系統的輸入阻抗屬於電感性電抗(inductive reactance)時，兩者關係為負相關。然而當系統的輸入阻抗落在電容性電抗(capacitive reactance)區時，其關係變成正相關，也因此會造成系統發生不穩定振盪現象。

關鍵字

電漿不穩定現象 ； 微波干涉儀 ； 射頻電漿阻抗計 ； 電漿量測

並列摘要

Plasma processing plays a crucial role in modern semiconductor manufacturing. The plasma processing results are determined by the plasma properties. Nowadays, the trend in the semiconductor industry to process smaller geometries as well as results in recipes being run at lower rf output power into lower pressures. Both factors result in the increase of probability of instability in a plasma source driven by rf power. In addition to the attachment-induced plasma relaxation oscillation occurring within this operating region, interactions between the power-dependent plasma impedance and load impedance-dependent rf delivery system also can introduce a notable system instability. This study focuses on the later one.
In this study, two diagnostic tools for characterizing the dynamic temporal behaviors of plasmas were developed. One is the rf impedance meter for measuring the rf power delivered to the inductive coil and the rf impedance of a ICP source. The other is the novel transmission-line microwave interferometer (TLMI) for plasma electron density measurements. However, the temporal variations of electron density are too subtle to detect by the TLMI when instability occurs at low power and low pressure plasma. Thus, in the experimental measurements of Ar instability, a rf-compensated Langmuir probe is used to carry out the measurements of plasma density.
In the second phase of this study, the performance of rf impedance meter is tested through the measurements of temporal electric characteristics of a pulsed Ar plasma. The time-resolved measurement results reveal that the magnitude of rf voltage and rf current increase as the duty cycles decrease at high modulation frequencies. A spike of the real part rf impedance due to the transition from capacitive to inductive coupling (E mode to H mode) is observed in the beginning of the modulation pulse. After the transition, the real part of the coil impedance increases and the imaginary part of the coil impedance decreases as plasma density rises. And the time dependences of ion saturation current follow the rf power closely. For plasmas with longer pulse periods, it needs about 0.7 ms to reach steady state at the setting of match network in this experiment.
In the study of Ar plasma instability, the measurement results reveal that the behaviors of instabilities will be distinct according to their matching settings. For the high frequency instability, the series capacitor (CL) position is fixed at where a perfect matching condition is achieved. The oscillation frequency is between 10 to 20 kHz and increases with the increasing of chamber pressure and rf power setting and the decreasing of parallel capacitor (CT) position. For the low frequency instability, the CL position is tuned at higher than the perfect matching point and the oscillation frequency is independent of operation condition and fixed at 150 Hz. In addition, oscillations of instability only occur as the CT position is higher than the perfect matching. From the disturbance simulation of the electric circuit model, a negative feedback mechanism between the electron density and the rf delivered power is obtained when a mismatch locates at the inductive reactance side of the Smith chart. As CT position increases from perfect matching point, the input impedance of system moves into the capacitive reactance region. The relation between the electron density and the rf delivered power becomes positive feedback. It leads system into unstable.

並列關鍵字

plasma instability ； ICP ； plasma diagnostic ； rf impedance meter ； microwave interferometer

參考文獻

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